Electrical impedance detection, as used in Electrical Impedance Mammography (EIM) and Electrical Impedance Imaging (EII), also referred to as Electrical Impedance Tomography (EIT), Electrical Impedance Scanning (EIS) and Applied Potential Tomography (APT), can provide an image of the spatial distribution of electrical impedance inside body tissue. This is attractive as a medical diagnostic tool because it is non-invasive and does not use ionizing radiation as in X-ray tomography or strong, highly uniform magnetic fields as in Magnetic Resonance Imaging (MRI).
Typically a two dimensional or three dimensional array of evenly spaced electrodes is attached to the body tissue about the region of interest. Voltages are applied across pairs of input electrodes, and output electric currents are measured at output electrodes. Alternatively, input electric currents are applied between pairs of input electrodes, and output voltages are measured at output electrodes or between pairs of output electrodes. For example, a very small alternating electric current is applied between one pair of electrodes, and the voltage between all other pairs of electrodes is measured. The process is then repeated with the current applied between a different pair of the electrodes.
The measured values of the voltage depend on the electrical impedance of the body tissue, and from these values an image is constructed of the electrical impedance of the body tissue. By performing a plurality of such measurements, both two dimensional and three dimensional images can be constructed. Spatial variations revealed in electrical impedance images may result from variations in impedance between healthy and non-healthy tissues, variations in impedance between different tissues and organs, or variations in apparent impedance due to anisotropic effects resulting for example from muscle alignment.
Tissue or cellular changes associated with cancer cause significant localized variations in electrical impedance, and electrical impedance images can be used to detect breast carcinomas or other carcinomas.
The electric current or voltage applied to the electrodes may have a broad range of different frequencies. Different morphologies that have insignificant impedance at one frequency may have a more significant variation in impedance at a different frequency. Signals with different frequencies may penetrate the object in different ways. For example, at one frequency a signal may penetrate most significantly through the inside of cells of body tissue (e.g. intro-cellularly) and at another frequency a signal may penetrate most significantly though spaces between cells of body tissue (e.g. extra-cellularly).
Ultrasound scanning typically involves using a hand-held ultrasound probe that includes an array of ultrasound transducers which both transmit ultrasound energy into body tissue to be examined and receive ultrasound energy reflected from the body tissue. To generate ultrasound energy, a driver circuit of a processing unit sends precisely timed electrical signals to the transducers. Part of the ultrasound pulses is reflected in the body tissue under examination and returns to the transducers. The transducers then convert the received ultrasound energy into electrical signals which are amplified and processed to generate an image of the examined region.
Electrical impedance detection can provide diagnostic information about body tissue, whereas ultrasound scanning can provide high resolution imaging of body tissue.